>>34
AB↑=p↑、AC↑=q↑とおくと
AG↑=(○○+○○+○○)/3=○p↑+○q↑
|AG↑|^2=(○p↑+○q↑)・(○p↑+○q↑)
=○*|p↑|^2+○*p↑・q↑+○*|q↑|^2
=○*5^2+○*5*3*cos120°+○*3^2